The electric current sensors capable to measure the DC content are basic components for control and monitoring used in DC industrial applications for example motor management, power supplies, welding supplies, electrolysis, galvanic metallization etc. The amplitudes of the current in such eventually other low-power applications can occur in the range from fraction of milliampere to hundreds of kiloamperes.
The sensors for small currents are mainly based on resistive shunt inserted into current path and the voltage drop on the shunt is the measure of the current flowing through this path. Disadvantage of this approach is the discontinuity in the current path and additional power loss in the shunt. Therefore this method is limited to fractions of kiloamperes. Disadvantage of the shunt method is the need of the current circuit disconnection for shunt installation.
Other method of the current measurement is based on utilization of the magnetic field created by the electric current as a measure of the current. The magnetic field of the current carrying conductor is concentrated via magnetic circuit made of soft magnetic material into gap containing a magnetic field sensor. The sensor can exploit various principles like Hall Effect, magnetoresistance, nonlinearity of magnetization characteristic of soft magnetic materials, high frequency impedance dependence on the magnetic field etc. As the magnetic field sensors are generally not linear in order to obtain higher accuracy the sensor must be equipped with additional electronic feedback control. This control system needs for higher measured currents high power. This known compensating method is based on measurement of the flux level in the transformer core and compensation of the flux originating from the primary current with a secondary current in the secondary winding and this way keeping the total flux level in the gap zero. In this case the compensating secondary current is scaled measure for the primary current. Disadvantage of this known technique is that it is sensitive to secondary current waveform distortion due to saturation of the transformer core. Disadvantage of the Hall sensor used as zero level sensor is its not negligible and unstable offset.
A similar method is described in U.S. Pat. No. 5,552,979. According to this known method the flux level in the transformer core is calculated from the voltage over the secondary winding, and is constantly controlled to be kept within positive and negative saturation.
U.S. Pat. No. 5,053,695 describes a circuit which periodically resets the flux level in the current transformer core from saturated to unsaturated state.
A similar method of controlling a current transformer is described in R. Severns, “Improving and simplifying HF direct current sensors”, Proceedings of the 1986 IEEE Applied Power Electronics Conference (APEC 86), pp. 180 183. This paper describes a method periodically driving a current transformer core in and out of saturation. This suggested technique improves the ability to measure current in any arbitrary direction, and a way of making the current transformer core go in to and out of saturation fast, for high and physically asymmetric magnetic fields induced by primary current in the primary winding, thereby greatly reducing the losses and increasing the ability to measure high currents.
Another method is described in U.S. Pat. No. 5,811,965. This method involves a constant application of an alternating voltage to the secondary winding of a current transformer. This guarantees that the transformer core is in the linear mode when the measurements are conducted.
U.S. Pat. No. 7,071,678 presents a method of controlling the flux density in a current transformer to keep the transformer core saturated between two consecutive measurements in a sampling measurement system. Saturation disables transformation of primary to secondary current, and thereby disables losses in the secondary circuit during this time. Both AC and direct currents are possible to measure. The use of saturation of the transformer core also permits the core to be designed physically small. In order to get effective and accurate low power consuming measurements, both the magnetic fields originating from primary current flowing in the primary winding and external magnetic fields must be symmetrically physically spread in the transformer core. This greatly limits the possible physical arrangement of the primary winding. The invention describes a method dividing the secondary winding into two or more separate winding sections.
Generally, all methods based on concentration of the magnetic field into place of a local magnetic field sensor via passive magnetic core suffer from non-linearity and finite permeability of the real core material and thus not negligible magnetic resistance leading to increase of the mass of the core with extreme cost and space consuming solutions and remarkable power consumption. The passive magnetic cores lead to remarkable measurement error at non-homogeneous magnetic field.
The use of an amorphous wire as a magneto-impedance element has been proposed in U.S. Pat. No. 5,994,899. A magneto-impedance element includes an amorphous wire having a spiral magnetic anisotropy. A dc-biased alternating current is supplied to the amorphous wire, whereby a voltage is produced between both ends of the amorphous wire. The amplitude of the voltage depends on externally applied magnetic field. This method is generally not linear and has strongly limited amplitude range.
Disadvantage of magneto-impedance based sensors generally is in dependence on stray field, temperature, part variation and the like and therefore the accuracy is strongly limited.
To state of the art belong also the transducers based on fiber-optic sensing elements that use the magnetic and electric fields surrounding the conductor to modulate the condition of light in optical crystals. The advantages of these systems are the intrinsic safety associated with fiber-optic and rejection of ambient electromagnetic interference. The disadvantages of the fiber-optic sensors are the effects of pressure and temperature gradients, mechanical vibrations, and other environmental noises that can alter the birefringence along the fiber, resulting to scale factor variation. Main disadvantage of the fiber-optic devices is their high cost.
From the present state of the art arises a need for a sensor capable to measure the electric current including its DC component with negligible influence of magnetic field distribution in the measurement area to measurement accuracy in high current amplitude range, without need for current circuit disconnection and negligible space requirement.